Armyworm: What to Know?

Medically Reviewed by Poonam Sachdev on January 16, 2023
Written by Venkat S.R.
5 min read

Armyworms are pests that attack cereal crops such as rice and grass pastures. Most armyworms cause damage to crops through defoliation — the premature removal of the grassy parts of plants such as leaves. In some cases, armyworms also cause seed damage in crops like wheat, barley, and oats.

On rare occasions, these pests “march” out from crops and pastures in large numbers to search for food sources. This is referred to as a plague, which is where the moniker “armyworms” originates. In this article, you will learn how to prevent armyworms and the best methods for preventing armyworm damage.

Armyworms are common caterpillars. They have four life stages: egg, larva, pupa, and adult. Adult armyworms are moderately sized with a wingspan of around 1.5 inches. Fall armyworms are gray, while true armyworms are tan in color with white spots in their underwings. Armyworm moths are active during the night and attracted to light. 

To begin mating, willing females release a sex pheromone to attract males, and then they lay eggs in areas with lush grass or weeds. The eggs are laid close to each other in tight clusters — there may be as many as 200 white or yellowish eggs in a single cluster which darken as they are about to hatch. Adult female armyworms are known to lay as many as 1,800 eggs over a few days. It takes around three to four days for the eggs to hatch under ideal circumstances. The eggs tend to hatch when the temperature is between 77°F and 84°F, and the chance of hatching reduces as the temperature drops.

Once the eggs hatch, the worms enter their larval stage which is when they are most destructive to crops. During this stage, armyworms have three pairs of true legs near the head and five pairs of fleshy legs — called “prolegs” — in the abdomen. Armyworms go through six larval stages, each called an instar. The sixth instar lasts around seven days and is responsible for roughly 80% of all foliage damage that occurs during the entire larval stage.

To transition to their next stage, larvae build pupal cells around themselves called a cocoon. After one to three weeks, adult moths break out of the reddish-brown pupal cell cocoon. The overall life cycle of the armyworm is 35-60 days.

Some of the common species of armyworms include:

  • Fall armyworm (Spodoptera frudiperda)
  • Yellow-striped armyworm (Spodoptera ornithogalli)
  • Beet armyworm (Spodoptera exigua)
  • True armyworm (Mythimna unipuncta)

Of these, fall armyworms are the pests most commonly found to damage turfgrass such as that on golf courses and other playing fields. There have been incidents in which the grass in areas as large as a football field has been eaten away in two to three days.

Armyworms are native to tropical regions in the Western Hemisphere including Argentina and the U.S. In the U.S., armyworms are a common sight in states east of the Rocky Mountains. Southeastern states face the brunt of the damage caused by these worms, but there were reports of armyworm sightings in West and Central Africa in 2016, indicating that these worms may become a cause for concern on the African continent as well.

Adult armyworms typically prefer plant nectar and are not considered a threat to crops. Larvae, however, feed on grass and sometimes target cereals such as small grains, corn, rice, and turfgrass. If there is not enough nutrition around them, larvae can also survive on a diet of broadleaf plant species.

Adult armyworms can’t survive in freezing winter temperatures, so they migrate each season. Because they are formidable fliers, they are able to cover large distances to reach areas with more suitable climates. The only areas in the U.S. that armyworms do not migrate from during the winter are southern Florida and southern Texas.

Armyworms prefer to feed on leaf tips and along the leaf margins, sparing only the midribs. You can confirm whether the plant has been attacked by armyworms by looking around the plant to check for the presence of the insect or its eggs. You can also determine the presence of armyworms according to these characteristics:

  • Larvae activity on plant crops leaves the heads of the cereals or oats on the ground. This is a tell-tale sign of larval infestation, as strong winds don’t sever the cereal heads completely off the plant stalk.
  • Chewing and nibbling patterns along the leaf edges are common indicators of armyworm activity. 
  • Caterpillar waste might also be present on the plant leaves or at the base of the plant. This waste looks like greenish or yellowish cylindrical pieces that are one to 2 millimeters in length.

Moths generally prefer dense areas to lay their eggs which is why areas like thick grass pastures and those along the fences are more vulnerable to armyworm infestations. When pesticides are sprayed to control weed growth, armyworms tend to move on to other food sources such as crop fields.

Moths generally prefer dense areas to lay their eggs, which is why areas like the thick grass pastures and those along the fences are more vulnerable to armyworm infestations. The presence of weeds in and around the fields is another factor that could potentially lead to armyworm infestations. When pesticides are sprayed to control weed growth, armyworms tend to move on to other food sources such as crop fields.

While the armyworm does not pose a direct threat when it comes to human health, there are indirect health and economic effects to be considered. The utilization of pesticides to control armyworm infestation could cause an increased intake of harmful chemicals and also lead to higher health expenses to overcome their ill effects. Crop damage due to armyworm infestation could also cause considerable financial loss to farmers.

Some organic insecticides like Bacillus thuringiensis, spinosad, pyrethrins, and horticultural oils are effective against armyworm infestations. Some synthetic insecticides containing active ingredients such deltamethrin, cyfluthrin, bifenthrin, and carbaryl are also effective.

Armyworm infestations pose major challenges, including financial loss and health concerns. Early detection is vital for effective armyworm treatment and control, especially when crops are about to be harvested. It takes skill and time to identify the extent of the infestation, but the effort is worthwhile. Although exhaustive, you can look at some of the below points to understand when your crop may need protection against armyworms:

  • When you find egg clusters in 5% of the plants
  • When 25% of the plants in your field are damaged
  • When find live larvae living in your field

It’s vital to take appropriate action before the larvae become larger because once this happens, it becomes more difficult to control the infestation. You can do this by spraying pesticides — roughly 30 gallons, or 114 liters, per acre. Keep in mind, pesticide use is governed by state laws. Make sure any insecticide or pesticide you use adheres to the laws of the state you live in.

Armyworm infestations can also be reduced by removing grass weeds from within the fields or around the borders of the fields. This reduces the chances of adult moths laying eggs around the field. It also prevents the migration of the larval armyworms into the field. If you notice larvae moving from the weeds into the fields, it may be time to use insecticides to prevent crop damage.